1. Field of the Invention
This invention relates to a continuous pressing machine which can mold a sheet of thermosetting resin reinforced by a paper, cloth, glass fiber or the like, or laminated thermoplastic resin sheets or plywoods.
2. Prior Art
As for a pressing, a batch type press of a single- or multi-daylight process has been used. Materials pressed by this type of press are, for example, papers, clothes, glass fibers or the like impregnated with a liquid thermosetting resin which are pressed at room temperatures or elevated temperatures, thermoplastic resin sheets or films for forming laminated sheets, wood boards with adhesive coated on their surfaces for forming plywood at elevated temperatures. In this press, the material to be pressed is interposed between upper and lower press plates or die plates and pressed therebetween at either room temperatures or elevated temperatures. It has been desired to effect such a pressing operation in a continuous manner so as to save labor, improve the productivity and achieve a uniformity of the quality of the finished product.
One such continuous press machine employs at least a pair of opposed endless steel belts instead of the above-mentioned pressure plates, each extending around a pair of drums. A material to be pressed is continuously passed between the pair of endless belts, and pressure is put to the material at either room temperatures or elevated temperatures.
The continuous press machine employs a pair of steel belts and a plurality of pairs of upper and lower opposed rollers which are mounted respectively on the back sides of these belts facing away from the material to be pressed between the belts. The pairs of upper and lower opposed rollers put compression pressures to the material through the belts, at a space between the neighboring pairs of rollers, and the belts are in stretched condition, thus imparting a certain degree of pressure to the material between each two adjacent pairs of upper and lower rollers. Alternatively, two groups of small-diameter rollers are mounted respectively on the back sides of the belts and urged to the back surfaces of the belts by flat sheets so as to bring the outer surface of the endless belt into contact with the material. The small-diameter rollers are rotated and thereby move at about a half of the moving speed of the belt. With this apparatus, however, the material to be pressed is subjected to different pressures when it passes through the opposed endless steel belts. In other words, the material is subjected to the pressures of greater magnitude at regions where the belts touch the rollers and also is subjected to pressures of less magnitude at the other regions of the endless belts. In addition, since the pressures which can be applied to the endless belts are limited, the applicable average pressure is only up to several Kg/cm.sup.2. Therefore, this pressing mechanism can not be applied when a pressure of not less than 10 kg/cm.sup.2 is needed, in such a case where the material to be pressed produces volatile substance when heated or cured.. And so, it has been necessary to apply a uniform fluid pressure to the back side of each endless belt. In order to produce such a uniform fluid pressure, a pressure chamber is provided in the space defined by each of the endless steel belts and is adapted to hold a fluid under pressure, the pressure chamber having an opening disposed in opposed, slightly spaced relation to the back side surface of each endless belt. In this case, it is necessary to provide seal means for preventing the pressurized fluid in the pressure chamber from leaking through a gap between the pressure chamber and the back side surface of the endless steel belt. For example, Japanese Patent Application Laid Open (Kokai) No. 54-13075 discloses one such seal means which comprises either a flexible seal member or a lubricous solid matter secured to a housing defining the pressure chamber, either of the flexible seal member and the lubricous solid matter being pressed against the endless steel belt so as to prevent the leakage of the pressurized fluid in the pressure chamber through the gap.
In the case where the pressed product must have a smooth surface, in a multi-stage press of the batch type, the material to be pressed is placed between each pair of die plates of stainless steel, aluminum or the like each having a polished working surface. The material is pressed by the die plates at elevated temperatures, so that the polished surface of the die plate is copied to the pressed material. In the case of the continuous pressing, a working surface (i.e., an outer surface) of each endless steel belt must be polished to a mirror surface so that the pressed material has a smooth surface.
In the case of such a continuous pressing, seal means secured to the pressure chamber housing is pressed to the back side surface of the moving endless steel belt (i.e., the surface opposite to the polished surface which is to be in contact with a material to be pressed). The endless steel belt is relatively thin, and therefore when the seal means is pressed into frictional, sliding contact with the back side surface of the steel belt, the polished outer surface of the steel belt is affected. Particularly when foreign matter is jammed between the seal means and the belt, stripe defects develop on the surface of the belt. Also, uneven forces are applied to the back side surface of the steel belt, stripe defects may appear on the polished surface of the steel belt. In addition, other factors, such as a variation in thickness of the steel belt in the direction of its width, an unevenness of the material to be pressed, an undulation of the steel belt in the direction of its width when subjected to tension, a variation in force of pressing of the seal means against the steel belt, deteriorate the polished surface of the steel belt when the steel belt is operated continuously for a long time.
The seal means has a contact portion which is held in contact with the back side surface of the endless steel belt, this contact portion being made of rubber, plastics material or the like so as to achieve a good sealing effect. The contact portion of the seal means is subjected to deterioration upon lapse of time, and as a result, part of the contact portion may be broken off due to the pressure of the pressurized fluid in the pressure chamber, the elevated temperature, and a bulging of the contact portion caused by the pressurized fluid, in which case the broken-off part of the contact portion is jammed in the gap between the back side surface of the endless steel belt and the seal means, so that stripe defect is formed on the steel belt in its longitudinal direction. Thus, the polished outer surface of the steel belt is adversely affected by this stripe defect and can not be used. As described above, with the seal means which is designed to be pressed into frictional sliding contact with the back side surface of the endless steel belt, a satisfactory product has not been obtained.